An important application of the proposed semiconductor laser is the use in an optical sensor module measuring distances and/or movements. Such an optical sensor module comprises at least one optical sensor including a laser having a laser cavity for generating a measuring beam, converging means for converging the measuring beam in an action plane and for converging measuring beam radiation that is reflected by an object into the laser cavity to generate a self-mixing effect and means for measuring the self-mixing effect, which means comprise a radiation-sensitive detector and associated signal-processing circuitry. Such an optical sensor module may be included in an optical input device that is based on the movement of an object and the device with respect to each other, but may also form part of measuring apparatuses of different types.
U.S. Pat. No. 6,707,027 discloses such an optical input device that uses the self-mixing effect. Laser self-mixing occurs if an external reflector, or object, is arranged in front of a laser so that an external laser cavity is obtained. In the case of an input device, movement of the device and the object, i.e. the reflector, which may be a human finger or a desk surface, with respect to each other causes tuning of the external cavity. Such tuning results in re-adjustment of the laser equilibrium conditions and thus in detectable changes in the laser output power. These changes, or undulations, are repetitive as a function of the displacement of the object over a distance equal to half the wavelength of the laser radiation along the axis of the light beam. This means that the laser undulation frequency becomes proportional to the speed of the external reflector.
A measuring device based on laser self-mixing shows high sensitivity, and thus accuracy, which can be attributed to the fact that reflected laser radiation re-entering the laser cavity determines the laser frequency and thus is amplified in the laser cavity. In this way, high receiver sensitivity is obtained without the use of additional means, like optical filters, or complex devices such as interferometers. An optical input device of this type equipped with two diode lasers allows measurement of movements of the device and the object with respect to each other in two mutually perpendicular (x- and y-) directions and any intermediate direction. Such a device can be used to navigate or move a cursor across a display panel, for example, to select an icon on the display.
U.S. Pat. No. 5,892,786 discloses an output control of a vertical microcavity light emitting device. This device includes a VCSEL-type diode laser embedded between two DBR stacks, wherein a phototransistor is embedded in one of the DBR stacks. With the output of the phototransistor, which measures the intensity of the optical field inside of the laser cavity, the output power can be controlled to achieve a constant level. The device of this document neither uses the self-mixing effect nor is it designed to convert measuring radiation from an object into a measuring signal. The heterojunction phototransistor of this module comprises a layer which includes a quantum well. This quantum well increases the wavelength selectivity of the phototransistor to detect only radiation having the desired laser wavelength of stimulated emission and not the broad wavelength range of spontaneous emission. Due to this quantum well, the gain of the phototransistor is large. Such a device, however, is not suitable for applications such as measuring distances and/or movements with high accuracy using the self-mixing effect.